The registry URL used is determined by the scope of the package (see
npm-scope(7)). If no scope is specified, the default registry is used, which is
supplied by the registry config parameter. See npm-config(1),
npmrc(5), and npm-config(7) for more on managing npm's configuration.

Does npm send any information about me back to the registry?

Yes.

When making requests of the registry npm adds two headers with information
about your environment:

Npm-Scope – If your project is scoped, this header will contain its
scope. In the future npm hopes to build registry features that use this
information to allow you to customize your experience for your
organization.

Npm-In-CI – Set to "true" if npm believes this install is running in a
continous integration environment, "false" otherwise. This is detected by
looking for the following environment variables: CI, TDDIUM,
JENKINS_URL, bamboo.buildKey. If you'd like to learn more you may find
the original PR
interesting.
This is used to gather better metrics on how npm is used by humans, versus
build farms.

The npm registry does not to correlate the information in these headers with
any authenticated accounts that may be used in the same requests.

Can I run my own private registry?

Yes!

The easiest way is to replicate the couch database, and use the same (or
similar) design doc to implement the APIs.

If you set up continuous replication from the official CouchDB, and then
set your internal CouchDB as the registry config, then you'll be able
to read any published packages, in addition to your private ones, and by
default will only publish internally.

If you then want to publish a package for the whole world to see, you can
simply override the --registry option for that publish command.

I don't want my package published in the official registry. It's private.

Set "private": true in your package.json to prevent it from being
published at all, or
"publishConfig":{"registry":"http://my-internal-registry.local"}
to force it to be published only to your internal registry.